Method and Apparatus for Liquefying a Natural Gas Stream

ABSTRACT

The present invention relates to a method of liquefying a natural gas stream, wherein the natural gas stream ( 10 ) is provided at a pressure of 10-80 bar, supplied to a gas/liquid separator ( 31 ), and separated into a vaporous stream ( 40 ) and a liquid stream ( 30 ). The vaporous stream ( 40 ) is compressed to a pressure of at least 70, 84 bar heat exchanged against the vaporous stream ( 40 ), and liquefied to obtain a liquefied natural gas stream ( 100 ).

The present invention relates to a method of liquefying a natural gas stream.

Several methods of liquefying a natural gas stream thereby obtaining liquefied natural gas (LNG) are known. It is desirable to liquefy a natural gas stream for a number of reasons. As an example, natural gas can be stored and transported over long distances more readily as a liquid than in gaseous form, because it occupies a smaller volume and does not need to be stored at high pressures.

Examples of known methods of liquefying gas are disclosed in U.S. Pat. No. 6,272,882 and DE 102 26 597 A1.

According to FIG. 1 of DE 102 26 597 A1 a natural gas stream having a pressure of 70-100 bar is expanded (Expander X) to a pressure range of 40-70 bar, cooled (Heat Exchanger E1) and fed to a Heavy Hydrocarbon (HHC) column (T1). A C₂-rich fraction taken from the overhead of the HHC column is further cooled (E2) and fed to a further column (D). The overhead stream of this further column (D) is pressurized (V) to a pressure in the range of 50-100 bar and subsequently liquefied.

A problem of the method according to DE 102 26 597 is that it is unnecessarily complicated. A further problem of the above method is that it needs a relatively high cooling duty in the heat exchanger(s) for liquefying the natural gas.

It is an object of the present invention to minimize the above problems.

It is a further object of the present invention to decrease the total duty of the heat exchangers used for cooling and liquefying the natural gas.

It is an even further object of the present invention to provide an alternative method for liquefying a natural gas stream.

One or more of the above or other objects are achieved according to the present invention by providing a method of liquefying a natural gas stream, the method comprising the steps of:

(a) providing a feed stream containing natural gas at a pressure of 10-80 bar, preferably 10-50 bar;

(b) supplying the feed stream provided in step (a) to a gas/liquid separator;

(c) separating the feed stream in the gas/liquid separator into a vaporous stream and a liquid stream, the vaporous stream being enriched in methane relative to the feed stream, and the liquid stream being reduced in methane relative to the feed stream;

(d) compressing the vaporous stream obtained in step (c) thereby obtaining a compressed stream having a pressure of at least 70, preferably at least 84 bar;

(e) liquefying the compressed stream obtained in step (d) thereby obtaining a liquefied natural gas stream;

wherein the compressed stream obtained in step (d), before it is liquefied in step (e), is heat exchanged against the vaporous stream obtained in step (c),

and wherein the pressure of the feed stream as provided in step (a) is not increased until the compressing in step (d).

It has surprisingly been found that using the method according to the present invention, a significantly increased recovery of compounds heavier than methane can be obtained. An important advantage of the present invention is that this can be achieved in a surprisingly simple manner.

A further advantage of the present invention is that an increased production of liquefied natural gas can be obtained using a given refrigeration power. Thus, for a given refrigeration power (e.g. using a given line-up comprising one or more cryogenic heat exchangers, compressors, etc.), the method according to the present invention provides more LNG than a known process. It has been found that according to the present invention increases in LNG product as high as 20% may be obtained, while keeping the refrigeration power constant.

It is noted that US 2004/0079107 A1 discloses the heat exchanging of a compressed stream against a vaporous stream obtained from a distillation column. However, US 2004/0079107 A1 teaches away from the present invention, as paragraphs [0032] and [0033] (while referring to FIG. 4) of US 2004/0079107 A1 suggest to perform the liquefaction at lower pressures. Thus according to US 2004/0079107 A1 it is suggested to heat exchange the vaporous stream obtained from the distillation column against a compressed stream which is at a relatively low pressure, which is contrary to the present invention.

According to the present invention the natural gas stream may be any suitable gas stream to be liquefied, but is usually obtained from natural gas or petroleum reservoirs. As an alternative the natural gas may also be obtained from another source, also including a synthetic source such as a Fischer-Tropsch process.

Usually the natural gas stream is comprised substantially of methane. Preferably the feed stream comprises at least 60 mol % methane, more preferably at least 80 mol %, most preferably the feed stream comprises at least 90 mol % methane.

Depending on the source, the natural gas may contain varying amounts of hydrocarbons heavier than methane such as ethane, propane, butanes and pentanes as well as some aromatic hydrocarbons. The natural gas stream may also contain non-hydrocarbons such as H₂O, N₂, CO₂, H₂S and other sulphur compounds, and the like.

If desired, the feed stream containing the natural gas may be pre-treated before feeding it to the gas/liquid separator. This pre-treatment may comprise removal of undesired components such as CO₂ and H₂S, or other steps such as pre-cooling, pre-pressurizing or the like. As these steps are well known to the person skilled in the art, they are not further discussed here.

The gas/liquid separator may be any suitable means for obtaining a vaporous stream and a liquid stream, such as a scrubber, distillation column, etc. If desired, two or more gas/liquid separators may be present.

The person skilled in the art will readily understand that the increase in pressure of the vaporous stream may be performed in various ways, provided that a pressure of at least 70, preferably at least 84 bar is obtained. Preferably, the pressure in step (d) is increased by compressing the vaporous stream in a compressor, thereby obtaining a compressed stream. To this end one or more compressors may be used.

Also, the person skilled in the art will understand that the liquefaction of the pressurized vaporous stream may be performed in various ways, e.g. using one or more cryogenic heat exchangers.

Further the person skilled in the art will readily understand that after liquefaction, the liquefied natural gas may be further processed, if desired. As an example, the obtained LNG may be depressurized by means of a Joule-Thomson valve or by means of a cryogenic turbo-expander. Also, further intermediate processing steps between the gas/liquid separation and the liquefaction may be performed.

Preferably in step (d) the pressure is increased to at least 86 bar, preferably at least 90 bar. Herewith the amount of LNG product obtained may be increased. As a result of the relatively high pressure used, the vaporous stream may be supercritical, depending on the prevailing pressure and the composition of the respective vaporous stream. Preferably the vaporous stream is supercritical, as this avoids phase changes in the liquefaction process.

Further it is preferred that the vaporous stream obtained in step (c) has a C₅ ⁺ content of below 0.5 mol %, preferably below 0.1 mol %. This minimizes operating problems in the downstream liquefaction unit. With “C₅ ⁺ content” is meant the content of hydrocarbon components having five or more carbon atoms.

Further it is preferred that the compressed stream obtained in step (d) is cooled, e.g. in an ambient heat exchanger. Further it is preferred that the compressed stream is heat exchanged against the vaporous stream obtained in step (c).

Also it is preferred that the feed stream, before supplying to the gas/liquid separator in step (b), is expanded. Preferably the feed stream is expanded to a pressure <(below) 35 bar.

According to a particularly preferred embodiment of the method according to the present invention, an expander for expanding the feed stream is functionally coupled to a compressor for compressing the vaporous stream. As a result, the power generated by the expander is used at least partially for driving the compressor to which it is functionally coupled. Hereby, the expander and compressor form a so-called “compressor-expander scheme”, as a result of which the energy consumption of the whole process is minimized. As the person skilled in the art will readily understand what is meant with a “compressor-expander scheme”, this is not further discussed here.

In a further aspect the present invention relates to LNG product obtained by the method according to the present invention, in particular liquefied methane.

In an even further aspect the present invention relates to an apparatus suitable for performing the method according to the present invention, the apparatus at least comprising:

means for providing a feed stream containing natural gas at a pressure of 10-80 bar, preferably 10-50 bar;

a gas/liquid separator for separating the feed stream into a vaporous stream and a liquid stream, the vaporous stream being enriched in methane relative to the feed stream, and the liquid stream being reduced in methane relative to the feed stream;

a compressor for increasing the pressure of the vaporous stream obtained in the gas/liquid separator to a pressure of at least 70, preferably at least 84 bar, thereby obtaining a compressed stream;

a heat exchanger for heat exchanging the compressed stream against the vaporous stream obtained from the gas/liquid separator; and

a liquefaction unit for liquefying an effluent from the heat exchanger having a pressure of at least 70, preferably at least 84 bar, the liquefaction unit comprising at least one cryogenic heat exchanger.

Preferably, the apparatus further comprises an expander for expanding the feed stream.

According to a particularly preferred embodiment, the compressor and expander are functionally coupled, thereby forming a so-called “compressor-expander scheme”.

Hereinafter the invention will be further illustrated by the following non-limiting drawing. Herein shows:

FIG. 1 schematically a process scheme in accordance with an embodiment of the present invention; and

FIG. 2 schematically a process scheme in accordance with another embodiment of the present invention.

For the purpose of this description, a single reference number will be assigned to a line as well as a stream carried in that line. Same reference numbers refer to similar components.

FIG. 1 schematically shows a base load liquefied natural gas (LNG) export process and an apparatus (generally indicated with reference number 1) suitable for performing the same. A feed stream 10 containing natural gas is supplied to a gas/liquid separator 31 at a certain inlet pressure and inlet temperature. In the embodiment of FIG. 1 the feed stream 10 is pre-cooled against a refrigerant in a heat exchanger 11. Typically, the inlet pressure to heat exchanger 11 will be between 10 and 80 bar (preferably <(below) 50 bar), and the temperature will be close to ambient temperature, usually between 5 and 50° C.

If desired the feed stream 10 may have been pre-treated before it is fed to the separator 31. As an example, the feed stream 10 may be expanded (as also shown in the embodiment of FIG. 2 hereafter; in expander 12).

As mentioned above, in the embodiment of FIG. 1, the feed stream 10 is pre-cooled against a refrigerant in a heat exchanger 11, or in a train of heat exchangers, for instance comprising two or more heat exchangers operating at different refrigerant pressure levels. The pre-cooled feed stream in line 20 is at a pre-cooling temperature that is lower than the temperature in line 10. The pre-cooling temperature is chosen to form a partially condensed feed stream 20. Further, the pre-cooling temperature is chosen to optimise a subsequent separation step in separator 31.

As mentioned above, stream 20 is fed to the gas/liquid separator 31. There the feed stream in line 20 is separated into a vaporous overhead stream 40 and a liquid bottom stream 30. The overhead stream 40 is enriched in methane (and usually also ethane) relative to the feed stream 20.

The bottom stream 30 is generally liquid and usually contains some components that are freezable when they would be brought to a temperature at which methane is liquefied. Separator 31 can be a separator vessel or a distillation column such as a scrub column, depending on the separation required to remove freezable components from the feed stream. Typically the freezable components are CO₂, H₂S and hydrocarbon components having the molecular weight of pentane or higher. These freezable components may also at least partially have been removed from the feed stream before entering the separator 31.

The bottom stream 30 may also contain hydrocarbons that can be separately processed to form liquefied petroleum gas (LPG) products.

Usually, the bottom stream 30 is subjected to one or more fractionation steps to collect various natural gas liquid products.

The overhead stream 40 is led through an effluent stream heat exchanger 41, where it is indirectly heated against a stream of about ambient temperature (stream 70). Stream 50, which is discharged from the effluent stream heat exchanger 41 is then compressed via compressor 51 or a train of two or more compressors. The compressed stream is discharged at a pressure above 84 bar into line 60. The pressure-increase in this compression step is chosen between 30 bar and 150 bar, depending on the choices of respectively the separation pressure and the liquefaction pressure.

Part of the heat added during this compression step is removed from stream 60 against the ambient, for instance using an air cooler 61 or a water cooler. The resulting ambient-cooled stream 70 is then led to the effluent stream heat exchanger 41 where it is cooled in indirect heat exchange with the cold overhead stream 40.

The cold stream 80 is then further cooled in one or more external cooling stages. This may include a pre-cooling stage, here depicted as heat exchanger 81. A train of subsequent heat exchangers may be employed instead.

A pre-cooled stream 90 is then further cooled into liquefaction in a liquefaction unit (generally indicated by reference number 5) at least comprising a main cryogenic heat exchanger 91. Any suitable type of heat exchanger may be employed. Here depicted is a cryogenic heat exchanger 91 operated by a mixed refrigerant, of which light and heavy fractions are first autocooled in tubes running parallel to the pre-cooled stream (not shown) and then expanded to the shell side via inlet means 95 and 96 respectively. The spent heavy and light fractions are drawn from the shell side of the main cryogenic heat exchanger 91 via outlet 97. The spent refrigerant in line 97 can be recompressed to form a liquid, or, in case of a mixed refrigerant, a mixed vaporous light fraction and liquid heavy fraction.

Referring again to stream 60, the liquefaction pressure is chosen to exceed a pressure of at least 70, preferably at least 84 bar, more preferably above 86 bar. As a result, the vapour in stream 60 may be in a supercritical condition.

As a next step, the liquefied stream leaving the main cryogenic heat exchanger 91 via line 100 is further cooled in a flash step wherein the pressure is let down via a valve or liquid expander 101. Suitably the pressure after expanding is about atmospheric. Expansion heat is extracted from the liquefied stream, so that the temperature is further lowered to a temperature under which the liquefied product remains liquid at atmospheric pressure. Flash gas 130, typically containing nitrogen and some methane, is separated from the stream 110 in flash tank 111. A part of the flash gas 130 can be employed as fuel gas for providing energy to the liquefaction process. The liquid part of stream 110 is discharged from the bottom of flash tank 111 in line 120. This can be stored and transported as LNG.

Table I gives an overview of the pressures and temperatures of a stream at various parts in an example process of FIG. 1. Also the mol % of methane is indicated. The feed stream in line 10 of FIG. 1 comprised approximately the following composition: 85% methane, 6% ethane, 4% propane, 2% butanes, 1% C₅ ⁺ and 2% N₂. Freezable components such as H₂S, CO₂ and H₂O were previously removed.

TABLE I Temperature Mol % Line Pressure (bar) (° C.) methane 10 37 32 85 20 36.8 −42 85 40 36.8 −42 90 50 36.4 38 90 60 86 125 90 70 85.9 40 90 80 85.5 −38 90 90 85.3 −50 90 100 85.0 −151 90 110 1 −161 90

FIG. 2 schematically depicts an alternative embodiment of the process according to the invention. In this embodiment, the feed stream 10 is expanded in an expander 12 to a pressure below 35 bar before entering the separator 31 as stream 25.

Preferably, the compressor train 51 uses expansion energy from at least expander 12. To this end at least one compressor of the compressor train 51 is functionally coupled to the expander 12 thereby forming a so-called “compressor-expander scheme”. Additional compression power may however be provided to achieve a pressure above 84 bar. Preferably, the additional compressor motor power consumed by the compressor 51 is chosen close to or identical to the power required by the refrigerant compressors so that identical drivers can be employed for both purposes thereby providing cost and maintenance benefits.

Table II gives an indication of decrease in cooling duty in the heat exchangers for cooling and liquefaction of the natural gas using the process as described in FIG. 1 according to the present invention. As a comparison the same line-up as FIG. 1 was used, but—in contrast to the present invention—no heat exchanging took place in heat exchanger 41. As shown in Table II the present invention results in a significantly decreased cooling duty of about 10%.

TABLE II FIG. 1 without heat exchanger 41 FIG. 1 (invention) (comparison) Cooling duty in 2.27 3.25 heat exchanger 81 [MW] Cooling duty in 6.38 6.34 heat exchanger 91 [MW] Total [MW] 8.65 9.59 

1. A method of liquefying a natural gas stream, the method comprising the steps of: (a) providing a feed stream containing natural gas at a pressure of 10-80 bar; (b) supplying the feed stream provided in step (a) to a gas/liquid separator; (c) separating the feed stream in the gas/liquid separator into a vaporous stream and a liquid stream, the vaporous stream being enriched in methane relative to the feed stream, and the liquid stream being reduced in methane relative to the feed stream; (d) compressing the vaporous stream obtained in step (c) thereby obtaining a compressed stream having a pressure of at least 70 bar; (e) liquefying the compressed stream obtained in step (d) thereby obtaining a liquefied natural gas stream; wherein the compressed stream obtained in step (d), before it is liquefied in step (e), is heat exchanged against the vaporous stream obtained in step (c), and wherein the pressure of the feed stream as provided in step (a) is not increased until the compressing in step (d).
 2. The method according to claim 1, wherein in step (d) the pressure is increased to at least 86 bar.
 3. The method according to claim 1, wherein the vapour stream obtained in step (c) has a C₅ ⁺ content of below 0.5 mol %.
 4. The method according to claim 1, wherein the compressed stream obtained in step (d) is cooled, before it is heat exchanged against the vaporous stream obtained in step (c).
 5. The method according to claim 1, wherein the feed stream provided in step (a), before supplying to the gas/liquid separator in step (b), is expanded.
 6. The method according to claim 5, wherein an expander for expanding the feed stream is functionally coupled to a compressor for compressing the vaporous stream in step (d).
 7. An apparatus for liquefying a natural gas stream, the apparatus at least comprising: means for providing a feed stream containing natural gas at a pressure of 10-80 bar; a gas/liquid separator for separating the feed stream into a vaporous stream and a liquid stream, the vaporous stream being enriched in methane relative to the feed stream, and the liquid stream being reduced in methane relative to the feed stream; a compressor for increasing the pressure of the vaporous stream obtained in the gas/liquid separator to a pressure of at least 70 bar, thereby obtaining a compressed stream; a heat exchanger for heat exchanging the compressed stream against the vaporous stream obtained from the gas/liquid separator; and a liquefaction unit for liquefying an effluent from the heat exchanger having a pressure of at least 70 bar, the liquefaction unit comprising at least one cryogenic heat exchanger.
 8. The apparatus according to claim 7, wherein the apparatus further comprises an expander for expanding the feed stream before it is supplied to the gas/liquid separator.
 9. The apparatus according to claim 8, wherein the compressor and expander are functionally coupled.
 10. The apparatus according to claim 7, wherein no compressor is present between the means for providing the feed stream at a pressure of 10-80 bar and the compressor for increasing the pressure of the vaporous stream.
 11. The method according to claim 1, wherein in step (d) the pressure is increased to at least 84 bar.
 12. The method according to claim 1, wherein in step (d) the pressure is increased to at least 90 bar.
 13. A method of liquefying a natural gas stream, the method comprising the steps of: (a) providing a feed stream containing natural gas at a pressure of 10-50 bar; (b) supplying the feed stream provided in step (a) to a gas/liquid separator; (c) separating the feed stream in the gas/liquid separator into a vaporous stream and a liquid stream, the vaporous stream being enriched in methane relative to the feed stream, and the liquid stream being reduced in methane relative to the feed stream; (d) compressing the vaporous stream obtained in step (c) thereby obtaining a compressed stream having a pressure of at least 70 bar; (e) liquefying the compressed stream obtained in step (d) thereby obtaining a liquefied natural gas stream; wherein the compressed stream obtained in step (d), before it is liquefied in step (e), is heat exchanged against the vaporous stream obtained in step (c), and wherein the pressure of the feed stream as provided in step (a) is not increased until the compressing in step (d).
 14. The method according to claim 13, wherein in step (d) the pressure is increased to at least 84 bar.
 15. The method according to claim 13, wherein in step (d) the pressure is increased to at least 86 bar.
 16. The method according to claim 13, where in step (d) the pressure is increased to at least 90 bar.
 17. The method according to claim 2, wherein the vapour stream obtained in step (c) has a C₅ ⁺ content of below 0.5 mol %.
 18. The method according to claim 11, wherein the vapour stream obtained in step (c) has a C₅ ⁺ content of below 0.5 mol %.
 19. The method according to claim 12, wherein the vapour stream obtained in step (c) has a C₅ ⁺ content of below 0.5 mol %.
 20. The method according to claim 1, wherein the vapour stream obtained in step (c) has a C₅ ⁺ content of below 0.1 mol %. 